Gap Light Analysis Mobile App

Analysis of hemispherical photographs for estimation of vegetation canopy cover

GLAMA - Gap Light Analysis Mobile Application (3.4 version) is a tool for analysis of hemispherical photographs taken in forests (user's manual: http://www.sci.muni.cz/botany/glama/). It supports calculation of Canopy Cover Index = simply the canopy cover estimated in phytosociology by visual observation.

The program can be used either for hemispherical, wide-angle and standard photographs (with known angle of lens view), which are analysed either from JPEG files in the lab or directly from a smartphone/tablet camera. It means the application can be simply used in field research, or it may help to analyse hemispherical photographs previously taken by a professional digital fish-eye camera. The program was adjusted for an easiest use in the field. Most parameters can be switched only once and any other sample may skip most of the pre-defined parameters. Therefore, the analysis of one photograph takes only a few seconds.

More detailed information about this application is published in: Tichý L., 2016, Field test of canopy cover estimation by hemispherical photographs taken with a smartphone. Journal of Vegetation Science, 27: 427-435.

Canopy Cover (Tree-Layer Cover) is the perpendicular projection of tree (and shrub) crowns onto a horizontal surface is called as canopy cover. Estimation of canopy cover by visual observation is widely used for description of vegetation structure in phytosociology and forest ecology.

Canopy Cover Index is the index used for more precise, non-unbiased estimation of canopy cover for sample plots used in phytosociology and community ecology vegetation survey. Hemispherical photograph is taken from one distinct position. Therefore, the raw fraction of dark pixels within some area represents real tree-layer cover only in limited central part of the photograph near a the zero zenith angle. Greater reduction of gap sizes and light intensity is usually found within increasing zenith angles. If we assume that the vegetation density is constant, the perpendicular projection of tree crowns to horizontal surface from any direction can be estimated as an exponential function of the cosine of the zenith angle (cos alpha). The light incoming from higher zenith angles lowers the precision of canopy cover estimation. Therefore, the weight of canopy cover calculated for partial area i is lower with increasing zenith angle (cos alpha).

The user of the GLAMA software must define, (1) which type of geometric lens distortion is used in a camera, (2) which is the minimum zenith angle of photograph edges and (3) which is a perimeter size (in pixels) of the whole hemisphere.